Gas turbine engine blades have airfoils that may be hollow and may include reinforcing ribs. These ribs may structurally reinforce the blade from several forces, including aerodynamic forces that tend to bend the blade about a base of the blade in a cantilever fashion, forces that tend to balloon a skin of the airfoil caused by higher static pressure present inside the hollow airfoil, and centrifugal force due to rotation of the blade. In addition to adding structural strength, in certain designs these ribs help define cooling channels present in the hollow airfoil.
Airfoils for gas turbine engine blades may be manufactured in various ways. One common way used is a casting process, due to its relatively low cost. In this process a casting core is first made using a rigid master die set. In this process a first half and a second half of the die are assembled together and form a hollow interior void. A casting core material is put into the hollow interior void and solidifies. Once solidified, the first and second die halves are separated by pulling them apart from each other along a straight separation line. The die halves are rigid, and the casting core is rigid. Consequently, there can be no interference between the casting core and the die halves as they are separated. This has resulted in casting core designs where any features in the casting core must be designed to permit the separation. For example, voids in the casting core, used subsequently to form the reinforcing ribs in the airfoil, are formed such that they are parallel to the direction along which the die halves are pulled apart. This necessarily results in the subsequently formed ribs being parallel to each other.
Certain airfoil designs include a twist in the airfoil from a base of the airfoil radially outward toward a tip of the airfoil. For any given radial cross section of the airfoil, a chord line connecting a leading edge of the airfoil to the trailing edge forms a chord line. A radially inward projection of the chord line forms an angle with a longitudinal axis of a rotor shaft of the gas turbine engine. When the angle formed changes from one radial cross section to the next in an airfoil, the blade may be considered twisted. While a casting process is able to accommodate a twist of the outer surfaces of the airfoil, the ribs must remain parallel to each other and to the separation line. As a result, in different radial cross sections the ribs will remain parallel to each other and the separation line, but since the airfoil is twisting, the ribs will change their orientation with respect to a skin of the airfoil. In certain circumstances it is preferred that the rib remain in the same (or similar) orientation to the skin in each cross section, such as for optimum strength, or optimum cooling when the rib defines part of a cooling channel. In certain circumstances it is preferred that the ribs not be parallel. Hence, other manufacturing techniques have been explored.
FIG. 1 shows a prior art airfoil disclosed in U.S. Pat. No. 4,512,069 to Hagemeister. In this twisted airfoil 10 a first rib 12 and a second rib 14 change orientation from a base cross section 16 to a tip cross section 18. This is accomplished by forging a worked conduit (drawn, swaged etc) into an untwisted airfoil shape and then twisting it. This working, forging, and twisting process is significantly different than casting, and may be more expensive.
A technique for forming ribs that are not parallel includes using two die halves and fugitive inserts. The fugitive inserts are positioned inside the hollow interior void, the casting material is placed in the hollow interior void, and the once the casting core is solidified the fugitive material is removed to form rib voids that are not parallel, and hence the subsequently formed ribs are not parallel.
However, these techniques may be costlier than simple casting, and hence there remains room in the art for improvement.